Silencing means for percussive pneumatic tools



23, 1968 A. D. BARBER ETAL 3,365,022

SILENCING MEANS FOR PERCUSSIVE PNEUMATIC TOOLS Filed Dec. 19, 1966 2 Sheets-Sheet 1 INVENTORS ANTONY D- BARBER Jan. 23, 1968 A. D. BARBER ETAL 3,365,022

SILENCING MEANS FOR PERGUSSIVE PNEUMATIC TOOLS 2 Sheets-Sheet 2 Filed Dec.

INVENTORS ANTQNY BAQBER BY M 4449, admw w r (v/041. We (1 ATTORNEYS United tates This invention relates to percussive pneumatic tools of the reciprocating piston and cylinder type, for example rock drills and roadbreakers, and is concerned with the provision of means for silencing such tools, that is, reducing substantially the noise of their operation.

The predominant noise produced by a pneumatic impact tool such as a roadbreaker or rock drill is that caused by the sudden expansion of compressed air exhausted from the working cylinder into the atmosphere, and to achieve a worthwhile reduction in noise it is necessary to find some means for reducing the impact of the emerging exhaust air stream.

Previous attempts at silencing have consisted mainly in the diffusion of the emerging exhaust air stream by passing it through a multiplicity of small holes or apertures. Compressed air however usually carries a proportion of water in suspension, and the cooling effect of the rapid expansion of the air stream when suddenly exhausted, not only causes this water to condense as droplets, but also tends to turn these droplets into ice which builds up on the outlets, impeding the air flow and so restricting the performance of the tool.

It is therefore necessary to maintain outlet passages of generous size which will not become restricted readily by ice, and the present invention provides a silencing arrangement which achieves this without passing the exhaust air stream directly into the atmosphere.

According to the present invention a percussive pneumatic tool of the type specified includes exhaust silencing means comprising first and second expansion chambers located outside the cylinder, the cylinder having a discharge port controlled by the piston and leading into the first expansion chamber and arranged to be uncovered at a late stage in the working stroke of the piston to allow the discharge of compressed air from the cylinder into the first expansion chamber, and transfer means controlled by the piston for placing the interior of the second chamber in communication with that of the first chamber at the same or a later stage in the working stroke of the piston to allow the discharge of the partly-expanded compressed air from the first expansion chamber into the second expansion chamber, the second expansion chamber having an open exhaust discharging into the atmosphere.

In one form of the invention the transfer means comprises an elongated passage of restricted cross-section formed in and passing around or through the piston and arranged to inter-connect the two expansion chambers at the said late stage in the working stroke of the piston.

This interconnecting passage may conveniently be afforded by a surface groove formed in the cylindrical flank of the piston and extending around it, the groove being arranged to overlap simultaneously a first transfer port in the cylinder wall leading into the first expansion chamber and a second transfer port in the cylinder wall leading into the second expansion chamber at the said late stage in the working stroke of the piston.

The first expansion chamber preferably has only a single port communicating with its interior, the single port constituting both the discharge port and the first transfer port, the single port being wide enough to overlap the ate r t ice groove in the piston at the same time as it is partly uncovered and open beyond the piston into the cylinder. In this way the re-entry of the partly-expanded exhaust air stream from the first expansion chamber is effected through the same discharge port through which it orig inally left the cylinder the discharge port being made wide enough for this purpose. Alternatively the discharge port might be duplicated to provide the desired wide overlap. The passage of the exhaust air stream through the two expansion chambers in turn, and through the restricted passage that connects them, before being released to the atmosphere, has proved to be an effective way of reducing the amplitude of the air pulsations above a certain frequency, dependent on the relative proportions of the expansion chambers and the restricted passage.

Moreover the invention has the effect of reducing the liability of ice formation in the exhaust passages, by avoiding a substantial temperature drop in the exhausted air when discharged into the atmosphere. The compressed air discharged from the cylinder into the first expansion chamber is not free to pass directly therefrom into the atmosphere and tends to be brought to rest in that chamber. Even if not brought completely to rest it will not develop the very high velocity which it would otherwise have reached if discharged directly into the atmosphere. Since the air will have expanded in the first expansion chamber without doing significant physical work and without developing kinetic energy, its temperature will remain nearly constant, in accordance with Joules Law. Thus the combined volume afforded by the working chamber of the cylinder and the first expansion chamber will contain, at any stage in the exhaust phase, a quantity of partly-expanded air at a reduced pressure but at a temperature only slightly below its original temperature before expansion. Similarly, in passing from the first expansion chamber into the second expansion chamber, and thence to the atmosphere, the exhaust air does not attain a very high velocity and therefore its temperature drop is correspondingly small. Thus the danger of icing at the final exhaust is reduced.

Moreover in the case of a double-acting tool the single wide exhaust port may be arranged to allow initial discharge of the exhaust and re-entry of the exhaust air stream from the first expansion chamber into the circumferential groove on the return stroke as well as on the working stroke of the piston.

In another form of the invention the transfer means comprises simply a transfer port in the wall of the cylinder leading into the second expansion chamber and arranged to be uncovered by the piston at a late stage in its working stroke (i.e., after the discharge port has been uncovered) to interconnect the two expansion chambers via the already uncovered discharge port and the interior of the working chamber of the cylinder above the crown of the piston. Such an arrangement is particularly suitable for use with a tool having a long stroke such as a roadbreaker, which allows the discharge port to be so positioned as to be uncovered by the piston and the first expansion chamber to be filled well before the transfer port is uncovered by the piston.

The invention may be carried into practice in various ways, but two specific embodiments and a modification of one of them will now be described by way of example with reference to the accompanying drawings, in which:

FIGURE 1 is a diagrammatic side view partly sectioned, of a pneumatic rock drill provided with one form of exhaust silencing means embodying the invention, the drill piston being shown at a late stage in its working stroke;

FIGURE 2 shows the silencing means of FIGURE 1 with the drill piston at a late stage in its return stroke;

FIGURE 3 shows a modified form of the embodiment 1% of FIGURES l and 2 with the drill piston at a late stage in its return stroke;

FIGURE 4 is a view in longitudinal section of part of a pneumatic roadbreaker incorporating another form of silencing means embodying the invention, and shown in its initial exhaust stage;

FIGURE 5 is a cross-section on the line V-V of FIG- URE 4; and

FIGURE 6 is a fragmentary view similar to FIGURE 4 of the silencing means but shown in the final exhaust stage.

In the embodiment of FIGURES 1 and 2, the invention is incorporated in a pneumatic impact rock drill 10 comprising a double-acting piston 11 which reciprocates in a cylinder 12 under the influence of compressed air supplied alternately to opposite ends oft-he cylinder under the control of a distributing valve mounted in the head13 of the drill. The spent air is exhausted from the cylinder 12 through a discharge port 15 in the side of the cylinder at a late stage in the working and return strokes of the piston 11 when the latter uncovers the exhaust port 15 to the operative end of the cylinder, thereby enabling an adequate pressure differential to be maintained across the piston during both of its strokes. The piston 11 is mounted on a piston rod 16 which extends through one end of the cylinder to the rotary holder for the drill steel 17 in the usual way.

Mounted externally on opposite sides of the cylinder 12 are two part-cylindrical, arcuate expansion chambers 20 and 21 each of which extends axially over substantially the whole length of the cylinder and ciroumferentially subtends about 150 at the axis of the cylinder. Access to the first expansion chamber 20 is provided by the discharge port 15 which is formed in the wall of the cylinder mid-way along its axial length. The piston 11 is formed with a circumferential groove 23 in its cylindrical surface mid-way between its ends, and the width of the discharge port 15 is such that when it is initially uncovered by the trailing edge of the piston 11 on either stroke, its other edge will still overlap the circumferential groove 23 in the piston. Thus the exhaust air stream will be initially discharged from the operative end of the cylinder 12 into the first expansion chamber 20 through the uncovered edge portion of the exhaust port 15, and after initial expansion therein the air stream will then return from the expansion chamber 20 into the groove 23 in the piston, re-entering the cylinder 12 through the opposite edge portion of the exhaust port 15 which then still overlaps the groove 23. The exhaust air stream will travel around the piston 11 in the groove 23 to a transfer port 25 formed in the other side of the cylinder wall, for example in this case diametrically opposite to the discharge port 15, this transfer port 25 giving access to the interior of the second eX- pansion chamber 21. The exhaust air stream will enter the second expansion chamber 21 via the transfer port 25, and after further expansion therein will escape to atmos phere through a discharge stub pipe 26 extending through one axial end wall of the second expansion chamber 21.

The discharge port 15, is necessarily wider than the transfer port 25, and the two ports 15 and 25 are so positioned relatively to one another in the axial direction of the cylinder 12 that exhaust air from the operative end of the cylinder is constrained to pass through the uncovered edge of the discharge port 15 into the first expansion chamber 20, rather than into the transfer port 25 direct, which would bypass the first expansion chamber 20 and the connecting passage constituted by the groove 23. The exhaust air may pass along the groove 23 on both sides of the piston between the discharge port 15 and the transfer port 25, with an effective attenuation of noise due to the appreciable length of each path along the groove.

It will be seen that the silencer construction described above enables effective silencing to be obtained without undue increase in the external diameter of the pneumatic tool 11], and operates on both the working and the return strokes. The expansion chambers 20 and 21 may be cast integrally with the wall of the main cylinder, or may be made of sheet metal welded onto the exterior of the main cylinder. Again, the silencer assembly might be fabricated separately as a one-piece or two-piece shell and detachably secured to the cylinder, with a suitable seal interposed. Where the silencer assemb y is formed separately from the cylinder of the tool, the two expansion chambers maybe made of a suitable plastics material instead of metal, for example of polyurethane synthetic resin which has good s0und-deadening qualities as well as being readily moulded and capable of resisting impact and pressure fluctuations.

The two expansion chambers 20 and 21 need not necessarily be located on diametrically-opposite sides of the cylinder 12 as described and illustrated but they would be formed and arranged in a variety of different ways, for example even disposed coaxially one within or above the other and completely surrounding the cylinder, the discharge port and transfer port being correspondingly located.

In a typical rock drill or other percussive pneumatic tool, the energy of the percussive action derives from the pressure and volume of the compressed air in the operative end of the cylinder during the working stroke of the piston. The compressed air admitted to the other end of the cylinder serves to return the piston for a subsequent power stroke and also, in the case of a rock drill, to provide the power for rotating the drill steel holder to turn the drill steel and its bit. The air pressure required for these duties during the return stroke is less than that required for the working stroke, and its volume may also be less. The noise generated by the discharge of the return stroke exhaust will thus normally be less than that generated by the working stroke exhaust, and accordingly an alternative path for the return stroke exhaust air may be provided in some circumstances.

Thus in the modification illustrated in FIGURE 3, the cylinder 12 is formed with the forward edge 15 of its discharge port 15 (i.e., the edge of the port 15 which will be uncovered first during the return stroke of the piston 11) substantially level with the corresponding edge 25 of the transfer port 25, so that both will be uncovered simultaneously by the piston 11 and some of the return stroke exhaust air will be discharged directly into the second expansion chamber 21, bypassing the first expansion chamber 20 and the groove 23, whilst the remainder will pass into the first expansion chamber 20 and thence via the circumferential groove 23 to the second expansion chamber 21 as before. This modified arrangement may be less effective in reducing noise in certain circumstances, but because of the lesser quantity of energy involved in the return stroke exhaust, it may nevertheless be satisfactory for use in some applications.

The embodiment of FIGURES 1 and 2 relates to a rock drill which is a relatively short-stroke implement, but the use of a silencing arrangement whose two expansion chambers are inter-connected by a restricted passage such as the circumferential groove in the piston may not prove so effective in a tool such as a roadbreaker which has a longer working stroke and fewer reciprocations per minute of the piston. With the latter form of tool it is found preferable to employ a different silencing arrangement in which the moving piston is employed to isolate the two expansion chambers for a portion of the working stroke but to allow them to communicate via the working chamber of the cylinder at a late stage of the working stroke to exhaust the spent compressed air.

Thus in the embodiment of FIGURES 4 to 6 a roadbreaker tool 50 comprises a cylinder 51 in which a free piston 52 can slide, and compressed air is admitted alternately to opposite ends of the cylinder 51 by means of a distributing valve 53. At the end of its working stroke the piston .52, moving in the direction of the arrow 54,

strikes a plunger 55 guided in a sleeve 56 and bearing against the head of a pick 57 detachabiy inserted in a carrier 58, so that the force of the impact of the moving piston 52 is transmitted via the plunger 55 to the pick 57.

The cylinder 51, which is of flattened form as shown in FIGURE 5, is surrounded by an external cylindrical shroud 60 divided by a partition 61 into two compartments constituting the expansion chambers 62 and 63. A wide discharge port 64 formed in the wall of the cylinder 5]. near its lower end leads into the first expansion chamber 62, and an arrow transfer port 6 5 located opposite the discharge port 64 leads from the cylinder into the second expansion chamber 63. The first expansion chamber 62 is closed apart from the discharge port 64, but the second expansion chamber is provided with one or more exhaust vents 66 discharging to the atmosphere.

As the piston 52 moves down the cylinder 5'1 during its working stroke, it reaches and covers both the ports 64 and 65. As the piston moves on, its rear edge initially uncovers the edge portion of the wide discharge port 64 as shown in FIGURE 4, keeping the transfer port 65 fully covered, and used compressed air from the working chamber of the cylinder enters the first expansion chamber 62 and partially expands therein, approximately isothermally. The piston continues its travel down the cylinder until it uncovers the transfer port 65, the wide discharge port 6 4 being then nearly fully uncovered as shown in FIGURE 6. Now the partly-expanded compressed air in the first expansion chamber 62 leaves through the discharge port 64, crosses the interior of the cylinder above the crown of the piston 52 and, together with air at the same pressure from the cylinder, leaves the cylinder through the transfer port 65 and expands still further in the second expansion chamber 63, from which it escapes to the atmosphere through the exhaust vent or vents 66.

During the return stroke of the piston 52 the compressed air from the cylinder will once again be discharged first into the expansion chamber 62 and subsequently, after the piston has travelled further up the cylinder, will be transferred into the second chamber 63 from which it will be exhausted to the atmosphere. In this case however, the distance which the piston has to travel between the instance at which it starts to uncover the discharge port 64 and the instant at which it starts to uncover the transfer port 65 is slightly less than on the downward working stroke, the transfer port being slightly nearer to the lower end of the discharge port than to its upper end as measured along the longitudinal axis of the cylinder.

What we claim as our invention and desire to secure by Letters Patent is:

1. A percussive pneumatic tool of the reciprocating piston and cylinder type, provided with an exhaust silencing means comprising first and second expansion chambers located outside the cylinder, the cylinder having a discharge port controlled by the piston and leading into the first expansion chamber and arranged to be uncovered at a late stage in the working stroke of the piston to allow the discharge of compressed air from the cylinder into the first expansion chamber, and transfer means controlled by the piston for placing the interior of the second chamber in communication with that of the first chamber at the same or a later stage in the working stroke of the piston to allow the discharge of the partly-expanded compressed air from the first expansion chamber into the second expansion chamber, the second expansion chamher having an open exhaust discharging into the atmosphere.

2. A percussive pneumatic tool as claimed in claim 1 in which the transfer means comprises an elongated passage of restricted cross-section formed in and passing around or through the piston and arranged to inter-connect the two expansion chambers at the said late stage in the working stroke of the piston.

3. A percussive pneumatic tool as claimed in claim 2 in which the interconnecting passage comprises a surface groove formed in the cylindrical flank of the piston and extending around it, the groove being arranged to overlap simultaneously a first transfer port in the cylinder wall leading into the first expansion chamber and a second transfer port in the cylinder wall leading into the second expansion chamber at the said late stage in the working stroke of the piston.

4. A percussive pneumatic tool as claimed in claim 3 in which the first expansion chamber has only a single port communicating with its interior, the single port constituting both the discharge port and the first transfer port, the single port being wide enough to overlap the groove in the piston at the same time as it is partly uncovered and open beyond the piston into the cylinder.

5. A percussive pneumatic tool as claimed in claim 1 in which the said transfer means comprises a transfer port in the wall of the cylinder leading into the second expansion chamber and arranged to be uncovered by the piston at a late stage in its working stroke to inter-connect the two expansion chambers via the already uncovered discharge port and the interior of the cylinder.

6. A percussive pneumatic tool as claimed in claim 1 which is double-acting, and in which the same discharge port and transfer means are arranged to operate to silence the cylinder exhaust during the return stroke of the piston in the same manner as during the working stroke.

References Cited UNITED STATES PATENTS ROBERT S. WARD, IR., Primary Examiner; 

1. A PERCUSSIVE PNEUMATIC TOOL OF THE RECIPROCATING PISTON AND CYLINDER TYPE, PROVIDED WITH AN EXHAUST SILENCING MEANS COMPRISING FIRST AND SECOND EXPANSION CHAMBERS LOCATED OUTSIDE THE CYLINDER, THE CYLINDER HAVING A DISCHARGE PORT CONTROLLED BY THE PISTON AND LEADING INTO THE FIRST EXPANSION CHAMBER AND ARRANGED TO BE UNCOVERED AT A LATE STAGE IN THE WORKING STROKE OF THE PISTON TO ALLOW THE DISCHARGE OF COMPRESSED AIR FROM THE CYLINDER INTO THE FIRST EXPANSION CHAMBER, AND TRANSFER MEANS CONTROLLED BY THE PISTON FOR PLACING THE INTERIOR OF THE SECOND CHAMBER IN COMMUNICATION WITH THAT OF THE FIRST CHAMBER AT THE SAME OR A LATER STAGE IN THE WORKING STROKE OF THE PISTON TO ALLOW THE DISCHARGE OF THE PARTLY-EXPANDED COMPRESSED AIR FROM THE FIRST EXPANSION CHAMBER INTO THE SECOND EXPANSION CHAMBER, THE SECOND EXPANSION CHAMBER HAVING AN OPEN EXHAUST DISCHARGING INTO THE ATMOSPHERE. 